TY - JOUR

T1 - Observing the emergence of a quantum phase transition shell by shell

AU - Bayha, Luca

AU - Holten, Marvin

AU - Klemt, Ralf

AU - Subramanian, Keerthan

AU - Bjerlin, Johannes

AU - Reimann, Stephanie M.

AU - Bruun, Georg M.

AU - Preiss, Philipp M.

AU - Jochim, Selim

PY - 2020/11/26

Y1 - 2020/11/26

N2 - Many-body physics describes phenomena that cannot be understood by looking only at the constituents of a system(1). Striking examples are broken symmetry, phase transitions and collective excitations(2). To understand how such collective behaviour emerges as a system is gradually assembled from individual particles has been a goal in atomic, nuclear and solid-state physics for decades(3-6). Here we observe the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The transition is signalled by the softening of the mode associated with amplitude vibrations of the order parameter, usually referred to as a Higgs mode(7). We achieve fine control over ultracold fermions confined to two-dimensional harmonic potentials and prepare closed-shell configurations of 2, 6 and 12 fermionic atoms in the ground state with high fidelity. Spectroscopy is then performed on our mesoscopic system while tuning the pair energy from zero to a value larger than the shell spacing. Using full atom counting statistics, we find the lowest resonance to consist of coherently excited pairs only. The distinct non-monotonic interaction dependence of this many-body excitation, combined with comparison with numerical calculations allows us to identify it as the precursor of the Higgs mode. Our atomic simulator provides a way to study the emergence of collective phenomena and the thermodynamic limit, particle by particle.

AB - Many-body physics describes phenomena that cannot be understood by looking only at the constituents of a system(1). Striking examples are broken symmetry, phase transitions and collective excitations(2). To understand how such collective behaviour emerges as a system is gradually assembled from individual particles has been a goal in atomic, nuclear and solid-state physics for decades(3-6). Here we observe the few-body precursor of a quantum phase transition from a normal to a superfluid phase. The transition is signalled by the softening of the mode associated with amplitude vibrations of the order parameter, usually referred to as a Higgs mode(7). We achieve fine control over ultracold fermions confined to two-dimensional harmonic potentials and prepare closed-shell configurations of 2, 6 and 12 fermionic atoms in the ground state with high fidelity. Spectroscopy is then performed on our mesoscopic system while tuning the pair energy from zero to a value larger than the shell spacing. Using full atom counting statistics, we find the lowest resonance to consist of coherently excited pairs only. The distinct non-monotonic interaction dependence of this many-body excitation, combined with comparison with numerical calculations allows us to identify it as the precursor of the Higgs mode. Our atomic simulator provides a way to study the emergence of collective phenomena and the thermodynamic limit, particle by particle.

KW - HIGGS

KW - SUPERCONDUCTIVITY

KW - MODE

U2 - 10.1038/s41586-020-2936-y

DO - 10.1038/s41586-020-2936-y

M3 - Journal article

C2 - 33239796

VL - 587

SP - 583

EP - 587

JO - Nature

JF - Nature

SN - 0028-0836

IS - 7835

ER -